A number of storage systems today offer what can be broadly referred to as logical copy operations. Logical copy operations are intended to provide fast replication of data. For example, IBM offers a logical copy operation entitled FlashCopy that is often used with IBM's virtual storage access method (VSAM) storage systems. A logical copy operation is a process that creates a logical copy of data by associating data stored at a source location (which is the data being copied) with a target location (which will become the copy of the data). The logical copy operation may establish the relationship between the source location and the target location with a logical operation at first; for example, the logical copy operation may establish a series of pointers to associate data at the source location with corresponding data at the target location. The data may then be physically copied from the source location to the target location as a background operation.
Logical copy operations often occur on the storage device hardware itself, without involvement by the host operating system. As a result, the data, and the organization of the data in the data set, cannot be changed during the logical copy operation. If changes are necessary, the logical copy operation is not an option, and the copy operation is executed by the host operating system using a standard copy operation using traditional input/output (I/O).
One situation where changes in the data set, or its organization, can cause problems arises with changes in attributes in a VSAM system. Supported control interval (CI) and control area (CA) sizes may change over time. When a data set with an obsolete CI or CA size is copied, FlashCopy is not available as the target data set (where the data will be copied to) will have to be allocated with the new, supported CI and CA sizes. The host operating system is required to rearrange the records in the CIs to fit within the new CI sizes and/or CA sizes.
As a result, when an administrator installs a new version of VSAM, jobs and operations that would previously run in minutes could take hours on the first run. The cause is the source data sets that used to be eligible for FlashCopy which now have to be copied using much slower standard I/O operations. This can cause the system to miss service level agreements (SLAs), backup windows, and other deadlines.